DRIVE A GT3 and GT3RS -then decide from there!
Maybe you like Porsche's (especially GT3) rich racing heritage or the fact that the GT3 is a homologation special, and homologation specials of any car are always awesome, but the NSX did have a lot of success in Japan's SUPER GT (JGTC) even before they lost most of the original unibody and went longitudinal motor configurations.
If both cars produced the same power and torque curves with the same rubber size, rating and brakes then on the turn my nod would go towards the NSX due to the better weight dist while turning.
Of course we'l never see a NSX with the same size brakes and tires, etc. as the GT3.....
Note regarding the braking: it's not just the brake size that is causing the GT3 to outbrake you, it's the rear bias that reduces brake fade / improves brake balance in the GT3. I'm sure the brake size and playing a factor, but dynamics of the vehicle are completely different. You can outbrake and get on the gas faster, resulting in better lap times (most skilled 911 drivers use the rear weight bias to "swing" the rear around a corner...it's also how most unskilled drivers end up wrapping their 911's around a tree).
I'm pretty sure both the NSX and the GT3 have a 40/60 weight distribution. A lot of the braking performance is also dependent on the suspension geometry of the car and how it loads all 4 tires under braking. The NSX does use a decent amount of rear brakes but even with the NA2 ABS system, the Porche's ABS is far superior.
As far as rotor size goes, there are a 14" front & rear brake package from Performance Friction for the NSX with a multi-pad caliper that's pretty trick. It's on the FXMD Time Attack record holding NSX and i'll have a set on my car soon. (The GT3 has ~ 15" front 13.7" rear rotors). Also keep in mind the NSX was originally designed with a 205 15" front wheel, so the front fender wells are tiny compared to the Porsche with 19's and 15" rotors. But with aftermarket parts you can get as much performance as you want.
You can fit 235s under the front of an NSX (same as GT3) and i'm not sure but I think you can get a 295 or 305 under the stock rear fender (like a GT3), but a lot comes down to balance of the platform and spring rates.
**At the end of the day, drive all of them, figure out what YOUR needs are, what appeals to you the most, if you will daily or track it, and go from there.
Having driven many GT3s, I'm pretty confident a 450whp NSX with 235/275 R-comps, a competent suspension & brakes, NSX-R bodywork, and the JDM gear ratio and a OS Giken LSD should have no issues with a GT3 or GT3RS on track (ya I think the NSXs chassis is that good). While it's not fair because it's 450whp vs. 450bhp, I feel it shouldn't be a problem.
If someone with a GT3/RS would like to test this, let me know. I wouldn't mind being proven wrong but worst case it would be close but I still might have to give the hand to the NSX.
The GT3 is an awesome car, and the RS is even more rare, beautiful, higher appreciation, wide-fenders, etc... If you have the funds I would jump right to an RS over a GT3 especially in this economy. From an investment point of view the NSX will be a loss compared to the Porsche -which has a dry sump!
While the GT3 is awesome, IMO its hard to beat a turbo NSX with a reliable well designed system and tune. This is why i'm building a turbo NSX. It's not for everyone but then again everyone has different needs or intended uses.
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